To maximize the lift of an aircraft, it is desirable to deflect the flow of fluid over an airfoil through the largest possible angle. In an aircraft, for example, when the airstream passes over an airfoil, the air velocity at the boundary layer decreases due to skin friction. If the flow is further decelerated by the pressure forces that result at high lift conditions, the airstream will separate from the airfoil, causing chaotic air flow, reduced airflow deflection and decreased lift.
It has been known in the art that boundary layer suction or blowing can delay boundary layer separation to a point further downstream. Such systems increase lift but are mechanically complex.
Another type of system that delays airstream separation is shown in Ellis, U.S. Pat. No. 2,899,150, which discloses an airfoil with cavities in the upper surface. In operation, vortices are formed and trapped in the cavities, allowing the airstream to pass over air instead of airfoil skin, resulting in reduced skin friction and increased air velocity near the surface of the airfoil. This in turn delays airstream separation and maximizes lift.
Although the cavities shown in Ellis increase the amount of lift, they also increase the amount of drag across the airfoil, a characteristic that is undesirable when an aircraft is cruising at high speeds. Thus there is a need for an apparatus that exposes a vortex generating structure to the air flow during high lift requirements, and conceals the vortex structure from the air flow when low drag is desired and little lift is needed.